Photocell voltage supply



Feb. 18, 1958 H. 'c. FAIIIQBIANKS 2,824,239

PHOTOCELL VOLTAGE SUPPLY Filed Nov. L6, 1954 AMPLIFIER non/mm 35 H Mi li Q2 amass -llL Z v j v I .9 ==-/|L8 I f Z' l J I 5 .7 4 POWER i SUPPLY I 50 55 I 49 51 f .54 f TO AC. SUPPLY g I I t; a a TOALL 0mm 5 HEATE E 2L '1 J Howard C! Fairbanks INVENTOR. BYM 9 QMYQWQ/ .ATT OHNEYS United States Patent: Qfifice 2,824,239 Patented Feb. 18, 1958 PHOTOCELL VOLTAGE SUPPLY Howard C. Fairbanks, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey This invention relates to an improved system for supply ing potential to a photocell used in conjunction with an amplifier. In the projection of sound moving pictures, it is common to convert the variations in the sound track into audible sound by means of a'beam of light projecting through the sound track onto a photocell, the output or" which is amplified by a suitable high gain amplifier, which in turn drives a loud speaker. Normally a power supply is provided with such a unit for supplying the operating potentials to the amplifier and to the photocell, it being customary to employ a rectifier type of power supply which is adapted to be connected to the usual alternating current supply lines. Most modern day amplifiers use vacuum tubes which are of the indirectly heated cathode type which have the inherent characteristic of requiring a rather considerable length of time for the cathode to get up to operating temperature. During this warm-up period very little plate current is drawn by such a tube. On the other hand the conventional A. C. to D. C. power supply employs a rectifier which is capable of passing considerable current very soon after operating potentials are applied thereto. The pulsating direct current appearing at the output of the rectifier must, of course, be filtered to smooth out the ripple therein, and a suitable filter is normally provided for this purpose. Such a filter normally includes one or more relatively high capacity condensers which, under the low current conditions obtaining during the warm-up period, tend to charge up to a voltage considerably higher than that which obtains during normal operation of the associate amplifier.

It is desirable, in order to obtain the maximum undistorted output from the photocell to utilize a photocell of the gas type and to operate this photocell as nearly as possible at its maximum rated potential. Normally this voltage is considerably lower than the plate voltage which is applied to the amplifier vacuum tubes and it has been customary, in the past, to obtain the anode voltage for the photocell from a voltage divider connected across the output of the power supply. However, due to the higherthan-normal voltage which obtains at the output of the power supply during the warm-up period, the voltage across the photocell will rise to a value considerably in excess of its maximum ratings, causing ionization of the gas within the photocell, which in turn causes the photocell to break into oscillation at an audio frequency. While the amplifier, due to the fact that its tubes are still not drawing full rated current, will not be operating at full gain, nevertheless the oscillation developed by the photocell will be amplified to a sufiicient extent to cause a most undesirable audio squawk to be emitted from the loud speaker. Furthermore, such ionization apparently causes a deterioration of the photosensitive surface of the photocell cathode which rapidly reduces the sensitivity of the cell to a point where its replacement is required.

It is an object of this invention to provide a common power supply for a photocell and associated amplifier so arranged that the voltage across the photocell will be prevented from rising to a value sufiicient to cause oscillation under the above circumstances.

A further object of the invention is to provide an arrangement for supplying anode potential to a photocell wherein the anode potential is obtained from a voltage divider connected between the positive side of the power supply, and the normally positive end of the conventional cathode biasing resistor for the power output stage of the amplifier.

Further objects and advantages of this invention will become apparent from the following description and claims especially when considered in the light of the accompanying drawing wherein the single figure is a schematic diagram of an audio-amplifying system incorporating my improved photocell voltage supply system.

As shown in the drawing a photocell 1 which may be the photocell of the sound system of a sound movie projector, for example, is connected to the input of a high gain amplifier 2, the output of which in turn drives a loudspeaker 3. Operating potentials for the photocell 1 and amplifier 2 are obtained from a conventional A. C. to D. C. rectifier type of power supply 4 which includes a filter 5 for smoothing out the A. C. ripple in the output of the power supply. A typical filter arrangement is shown in the drawing and may, for example, comprise a pair of resistors 6 and 7, connected in series, and a plurality of filter condensers 8, 9 and 10 connected from the ends of the resistors to the negative side of the power supply, which is shown as being grounded. As is likewise conventional the input stage 11 of the amplifier, which operates at extremely low input-signal levels, ob-

. tains its positive potential from the point 12 onthe filter,

most .remote from the rectifier. Thus the minimum amount of ripple will be applied by the power supply to this stage. As is likewise common practice, the output stage 13 obtains its anode potential from the point 14 at the input end of the filter. The intermediate amplifier stage or stages 15 may, in accordance with usual practice, obtain positive operating potentials from an intermediate point 16 on the filter.

The amplifier input stage 11 is conventional and employs a triode 17 having its anode 18 connected through load resistor 19 to the positive terminal 12, while its cathode 20 is connected through a biasing resistor 21 to the negative side of the power supply. The grid 22 of this tube is connected through a coupling condenser 23 to the anode 24 of photocell 1, the cathode 25' of which is also connected to the negative side of the power supply.

The number of intermediate amplifier stages, and the number and type of tubes used in the output stage 13 will, of course, depend upon the amont of gain required and the audio output power desired at the speaker. For purposes of illustration output stage 13 is shown as comprising a pair of power output tubes 25, 26 of the beam power type, such as type 6L6, connected as a push-pull, class AB amplifier. The anodes 27, 28 of tubes 25 and 26 are connected to opposite ends of the center tapped primary of an output transformer 29, the center tap 30 of which, is connected to the screens 31 and 32 and thence to the positive terminal 14 on the power supply. The cathodes 33 and 34 are, as is usual, connected to the negative side of the power supply through a cathode biasing resistor 35.

Positive potential for the anode 24 of the photocell 1 is obtained from a voltage dividing arrangement which includes resistors 36, 37 and 38 connected in series from the point 12 on the power supply to the cathode end 39 of the cathode biasing resistor 35 of the power output stage 13. Resistors 36 and 37 are .of relatively high resistance and are so proportioned that the voltage at the point 40 between these resistors is, under normal operating conditions, substantially at the rated anode voltage for the photocell 1. Point 40 is connected to the anode of the photocell by means of high resistances 41 and 42. Since, at high level audio conditions, the voltage at the point 39 will tend to fluctuate slightly at audio frequencies, condenser 43 is connected from the point 44 between resistors 41 and 42 directly to the negative side of the power supply, while an additional condenser 45 is similarly connected from the junction 46 of resistors 37 and 38 to the negative side of the power supply. These condensers 43 and 45 together with-the resistors 41 and 38 serve as decoupling filters to prevent feedback of the audio frequency voltage variations from the cathodes of the tubes 25 and 26 to the input of the amplifier.

The power supply itself may be of any well known construction, the specific arrangement used depending upon the voltages and currents required by the particular amplifier used. In the instant case it is shown as comprising a transformer 47 having its primary 48 adapted to be connected by a suitable control switch 49 to the usual A. C. supply lines. The transformer 47 includes a conventional high voltage winding 50, the center tap 51 of which is grounded to serve as the negative side of the power supply and the ends of which are connected to the plates of a full wave rectifier 52. An additional secondary winding 53 on the transformer serves to supply a low voltage alternating current to the filament 54 of the rectifier 52, one side of which 55 serves as the positive output terminal from the rectifier and is connected to the input terminal 14 of the filter 5. A third secondary winding 56 serves to provide a low voltage alternating current to the heaters for the various vacuum tubes used in the amplifier, heaters 20', 33' and 34, which are associated with the cathodes 20, 33, and 34 in the input and output stages, being shown connected in parallel across this secondary winding 56.

Resistances 36, 37, and 38 constitute a voltage divider extending from the positive side of the power supply to the cathode end of the cathode resistor 35 of the power output stage 13, anode 24 of the photocell 1 obtaining its positive potential from an intermediate point 40 on this voltage divider. Alternatively resistors 36, 37, 38, and 35 may be considered as a voltage divider arranged across the output of the power supply with one tap 40 connected to the anode of the photocell and a second tap 39 connected to the cathodes of the power output stage 13.

In operation, when switch 49 is first closed rectifier 52 will rapidly become conductive since it uses, as is customary, a directly heated filament as its cathode. Thus the condensers 8, 9 and 10 in the filter will tend to charge up relatively rapidly to the peak value of the pulsating direct current passing through the rectifier. While the amplifier tubes at this time also have voltage applied to their heaters, since these tubes are of the indirectly-heated cathode type, they will not come up to operating temperature nearly as rapidly as does the rectifier 52 with the result that the plate current drawn by these tubes will rise but slowly from zero to its normal operating value. During this warm-up period the current drawn from the power supply will therefore not be sufficient to prevent the voltage across the condensers 8 to from rising appreciably above its normal value. As a result the voltage at the positive end 12 of the voltage divider will also be considerably above its normal value. If the other end of the voltage divider were connected directly to the negative side of the power supply, as is usual practice, the voltage at the photocell anode supply point 40 would likewise be proportionally above its normal operating value, with the result that, as previously explained, the photocell would break into a relatively high frequency audio oscillation. However, since the output tubes 25 and 26 are at this time drawing little or no current the voltage at the point 39 will be substantially at ground potential, which is several volts (about 22 volts, in the case of 6L6s) below the normal operating potential at this point, thus tending to automatically compensate for the higher-than-normal voltage at point 12. r

.4 As a result the voltage at point 40 will remain substantially at its normal value. As the amplifier tubes warm up and draw more and more plate current, the voltage at the point 39 will gradually increase toward its normal operating value. At the same time the increasing current being drawn from the power supply will produce a corresponding reduction in the voltage at point 12. Thus, by proper selection of components, the voltage applied to the anode of the photocell may be maintained very close to its desired value throughout the entire warm-up period.

The values of the various resistors used will, of course, depend upon the voltage characteristics of the power supply and the type and number of the various tubes used in the amplifier. It should be pointed out that resistors 36 and 37 in particular are preferably of much higher resistance than the cathode resistor 35. For example in an amplifier of the type shown, using 6L6s, cathode resistor 35 may have a value of only about 300 ohms, while resistors 36 and 37 may be respectively 430,000 ohms and 130,000 ohms. Thus the major variation in voltage across the resistor 35 is produced by the plate current drawn by the output tubes, the relatively small current flowing from terminal 12 through resistors 36, 37 and 38 and thence through resistor 35 having negligible effect on the voltage at the point 39. As previously mentioned, feedback of any audio voltage from point 39 to the input of the amplifier is effectively prevented by the decoupling filters 38, 45, and 41, 43.

While the invention has been illustrated as being embodied in an audio amplifying system of particular design, this has been done for purposes of illustration only and it is obvious that the principle of the invention can be readily applied with equal facility to photocell amplifiers of other types or which are intended for other uses. Likewise, while a transformer type of power supply has been illustrated, the same operation will obtain in the case of a transformerless power supply of a so-called universal type, or of any type of power supply wherein the output voltage tends to soar above normal during the warm-up period of the associated amplifier. Thus it is obvious that many changes can be made in the individual components without departing from the spirit and scope of the invention as defined by the appended claims.

. What I claim is:

1. In combination with an electronic amplifier including a power output tube having an indirectly heated cathode, and a power supply for applying D. C. operating potentials to said amplifier and including a rectifier tube of the filament cathode type whereby upon the application of power to the input of said power supply said rectifier will become conductive before said output tube, said amplifier including a cathode resistor in series with said output tube between its cathode and the negative side of said power supply, a photocell connected to the input of said amplifier and including an anode, and means for applying a positive potential to the anode of said photocell comprising a resistance voltage divider connected between the positive side of said power supply and the cathode end of said resistor, and a direct current connection from said photocell anode to an intermediate point on said voltage divider.

2. In combination with a photocell having an anode, and an amplifier for amplifying the output of said photocell and including at least an input tube and a power output tube, said tubes having indirectly heated cathodes and heaters therefor, said tubes, upon energization of their heaters, being incapable of passing normal current until the elapse of a warm-up period of appreciable duration, means including a power supply adapted upon energization to simultaneously apply D. C. potentials to said tubes and said photocell and to energize said heaters, and comprising a cathode biassing resistor. connected between said output tube and the negative side of said power supply, a positive supply terminal for said input stage and a voltage dropping resistor connected between said rectifier and said positive supply terminal, a filter condenser connected between said positive supply terminal and the other side of said power supply, said power supply including a rectifier for providing D. C. potentials, said rectifier being of the type which, upon energization of the power supply, becomes conductive considerably before the end of said warm-up period whereby the voltage at said positive supply terminal rises during said warm-up period to a value considerably above the normal operating value, a resistance voltage divider connected between said positive supply terminal and the posititve end of said cathode biassing resistor, and means connecting an intermediate point on said voltage divider to the anode of said photocell.

3. In combination with an electronic amplifier including a power output tube having an indirectly heated cathode and a heater therefor, and a power supply adapted upon energization to supply D. C. potentials to said amplifier and" to energize said heater, said amplifier, including said tube, being incapable of drawing normal current from said power supply during a predetermined warm-up period of considerable duration following energization of said power supply, said power supply including a rectifier, said rectifier being of the type which, upon energization of said power supply, becomes conductive considerably before the end of said warm-up period Whereby the D. C. potentials supplied by said power supply to said amplifier rise during said warm-up period to values considerably above their normal operating values, said amplifier including a cathode resistor in series with said output tube between its cathode and the negative side of said power supply, a photocell connected to the input of said amplifier and including an anode, and means for applying a positive potential to the anode of said photocell comprising a resistance voltage divider connected between the positive side of said power supply and the cathode end of said resistor and a direct current connection from said photocell anode to an intermediate point on said voltage divider.

4. In combination with an electronic amplifier having an input tube and a power output tube, said tubes having indirectly heated cathodes and heaters therefor, said tubes, upon energization of their heaters, being incapable of passing normal current until the elapse of a warm-up period of appreciable duration, power supply means for supplying operating potentials to said amplifier including a rectifier for supplying D. C. potentials to said tubes, a filter connected to the output of said rectifier, connections from said filter to said tubes, and means for simultaneously applying A. C. energy to said rectifier and to said heaters, sai'd rectifier being of the type which, upon such application of A. C. energy thereto, becomes conductive considerably before the end of said warm-up period, whereby the D. C. potentials delivered to said amplifier rise during said warm-up period to values above their normal operating values, said amplifier including a cathode resistor in series with said output tube between its cathode and the negative side of said power supply, a photocell connected to said input tube including an anode, and means for applying a positive potential to the anode of said photocell comprising a high resistance voltage divider connected from the positive output side of said filter to the cathode end of said resistor, and a direct current connection from said photocell anode to an intermediate point on said voltage divider.

Timmer July 12, 1938 Lobosco Sept. 23, 1952 

